In a radio communication system in which at least one party of an established telephone connection is mobile, the transmitted radio signals are subjected to so-called Rayleigh fading. This means that the radio wave transmitted directly from a transmitter to a receiver will interfere with a large number of reflected waves, for instance waves that have been reflected by buildings, hills and vehicles, for instance. One characteristic feature of such fading is that the strength of the signal received can be very low over short periods of time, although still have a high mean value. Similarly, the strength of the signals received may be very low when communicating between two stationary stations. Consequently, a connection may be interrupted or broken when a mobile station, for instance a vehicular mobile telephone stops at a location in which the received signal strength is low.
When the wavelength is short, for instance at the frequency of 900 MHz, two antennas located only a short distance from one another may exhibit totally different fading patterns. For instance, the received signal strength in the one antenna may be high while, at the same time, the signal strength in the other antenna is low, whereas the conditions may be the reverse only a short time thereafter. In so-called diversity reception, there is used at least two receiver antennas and the signals from these antennas are added in some suitable manner, or alternatively only the strongest signal at that moment in time is chosen. The procedure of adding several signals in a manner such that the signals will coact mutually and not extinguish one another is complicated, however.
EP, A2, 0 318 665 teaches two methods of selecting one of two receiver antennas continuously. According to one method, the strength of the signal received on the antenna used during a time slot in which transmission is effected to the receiver concerned is measured and stored. During a following time slot, when transmission is effected to another receiver, the strength of the signal for the other antenna is measured, and the measured value is then compared with the stored value of the antenna previously used. The antenna to be used during the next time slot, where transmission is to be effected to the receiver concerned, is then chosen on the basis of the highest value obtained.
According to another method, the signal strengths of both antennas are measured during a time slot in which transmission is effected to another receiver, whereafter the antenna having the highest signal strength is used during the time slot in which transmission shall be effected to the receiver concerned.
When antenna selection is effected with the aid of known methods, there is a danger that the signal strength of one antenna will be measured at precisely that moment when signal strength is very low. There is therefore a risk that a less suitable antenna will be selected, i.e. the antenna selected will not be the antenna which would provide the highest signal strength during the time slot in which transmission shall take place to the receiver concerned.